The primary medical instrument used to monitor the electrical activity of a patient's brain is the electroencephalograph (EEG). EEGs monitor brain activity by measuring the very small voltage fluctuations that are generated in the brain, which are detected by electrodes attached to a patient's scalp. To aid in studying these analog signals, a record of the voltage fluctuations (called an electroencephalogram) is often made over time. Traditionally, electroencephalograms are made using a mechanical EEG recorder that employs pens to record the analog voltage fluctuations on a strip of paper. As a continuous chart of paper is moved beneath an array of galvanometer-driven ink pens, the pens trace out the brain wave activity as a series of wavy or jagged lines.
Medical personnel analyze EEG waveforms (electroencephalograms) representing neural activity at various points on a patient's brain when monitoring, diagnosing, and treating the patient. Of particular importance to medical personnel are dominant frequency components, e.g., the modal frequency, of portions of the EEG waveforms. Medical personnel typically need to know the average frequency and a representative amplitude of the dominant frequency components of the various EEG waveforms. This frequency and amplitude information allows, for example, medical personnel to compare neural activity at various points on the patient's brain. With traditional paper EEG recorders, medical personnel measure frequencies and amplitudes using a transparent plastic ruler that has several frequency scales printed on it. The ruler is superimposed over the waveforms on the paper, the closest matching frequency scale is aligned with the waveform, and the frequency is that printed on the ruler. The ruler may have another scale for measuring the distance between a peak and valley of the waveform so as to determine a representative amplitude. Similar measurements can be taken on various waveforms for making comparisons. For the ruler to provide valid measurements, the frequency scales printed on the ruler must be calibrated for a particular paper speed (e.g., 30 mm/sec.) of the EEG recorder, and the amplitude scale must be calibrated the same as the vertical displacement calibration of the EEG recorder (e.g., 7 .mu.volt/mm).
Recently, work has begun on digital EEG recorders to replace analog EEGs and their associated mechanical pen-on-paper recorders. Instead of printing the EEG waveforms on paper, digital EEG recorders convert sensed analog waveforms into digital signals that are stored in some digital storage medium such as random access memory (RAM), hard disks, storage tapes, etc. The stored digital waveforms can then be transferred to a digital EEG reader for display and analysis by medical personnel. A digital EEG reader can consist of a stock personal computer including memory, a processor, input devices and an electronic display screen, e.g., a cathode ray tube (CRT) monitor, for displaying the waveforms. On such display screens, a transparent plastic ruler as used in the past with paper EEG recorders can conceivably be used to measure frequency and amplitude. Unfortunately, however, if the display screen size is changed or if a different display calibration is used, the same plastic ruler will no longer be properly calibrated. Furthermore, the use of a plastic ruler on a display screen can be cumbersome.
With the introduction of digital EEG recorders and readers, an easier and more flexible approach is desirable and feasible. In fact, presently there are electronic systems that provide waveform frequency and amplitude measurements. However, the measurements provided by presently available systems are not well suited for EEG measurements. Most systems perform complex mathematical analyses (i.e., Fourier transforms) that give the entire frequency spectrum of the EEG waveform. In other words, such mathematical analyses give the amplitudes and frequencies of a band of frequencies that form an EEG waveform. The majority of this information is not useful and, many times, not accurate as the most significant frequencies tend to be spread out over a band of frequencies.
Rather than all this complex information, what is most significant is the average frequency and a representative amplitude of a dominant frequency component (e.g., the modal frequency). Medical personnel can visually pick out significant frequency components. A simple and flexible tool that allows medical personnel to measure a representative amplitude and the average frequency of visually selected frequency components is needed. While method and apparatus do currently exist for electronically measuring the frequency of a selected frequency component, the measurements can only be based on a single cycle of the frequency component. This makes the measurements very sensitive to operator error and minor fluctuations in waveforms.
The present invention is directed to overcoming the foregoing problems by providing a method and apparatus that allows medical personnel to electronically measure the average frequency and a representative amplitude of a desired frequency component of an EEG waveform using conventional computer input devices. The method and apparatus is self calibrating. As a result, the same tool can be used to easily measure frequencies and amplitudes on various monitor sizes using various display calibrations.